1. Field of the Invention
The present invention relates to a variable geometry system turbocharger equipped with a variable nozzle unit that can change a passage area for (a flow rate of) an exhaust gas to be supplied to a turbine impeller, and a method of manufacturing a housing for a variable geometry system turbocharger.
2. Description of the Related Art
In recent years, various developments have been made on a variable nozzle unit to be installed in a variable geometry system turbocharger. A general configuration of a variable nozzle unit is as described below.
Inside a turbocharger housing of a variable geometry system turbocharger, a base ring is provided concentrically with a turbine impeller. Multiple support holes are formed in the base ring at regular intervals in its circumferential direction to penetrate the base ring. Moreover, multiple variable nozzles are disposed on the base ring at regular intervals in its circumferential direction in such a way as to surround the turbine impeller. Each variable nozzle is rotatable about its shaft center which is parallel to the shaft center of the turbine impeller. Moreover, a nozzle shaft is formed integrally with a side surface of each variable nozzle on one side in the axial direction of the turbine impeller. Each nozzle shaft is rotatably supported by the corresponding support hole in the base ring.
A link mechanism for synchronously rotating the multiple variable nozzles is disposed in an annular link chamber that is defined on the one side in the axial direction of the base ring inside the turbocharger housing. The link mechanism is connected to and interlocks with the nozzle shafts of the multiple variable nozzles. When the multiple variable nozzles are synchronously rotated in a forward direction (an opening direction), a passage area for an exhaust gas to be supplied to the turbine impeller side is increased. On the other hand, when the multiple variable nozzles are synchronously rotated in a reverse direction (a closing direction), the passage area for the exhaust gas is decreased.
In general, the exhaust gas contains water vapor. When the engine stops its operation, the exhaust gas containing the water vapor is retained in the link chamber. Accordingly, when the variable geometry system turbocharger is used in cold weather, the water vapor is condensed (liquefied) and frozen inside the link chamber. If part of the link mechanism is immersed in the condensate water when the water gets frozen, the link mechanism will not operate smoothly when the engine resumes its operation. In other words, the freezing of the condensate water poses a problem for the operation of the variable nozzle unit. A variable geometry system turbocharger described in Japanese Patent Application Laid-Open Publication No. 2012-102660 (JP 2012-102660A) discloses a configuration for overcoming such a problem.